A sharp increase in the frequency
of earthquakes near Fox Creek, Alberta, began in December 2013 in
response to hydraulic fracturing. Using a hydraulic fracturing database,
we explore relationships between injection parameters and seismicity
response. We show that induced earthquakes are associated with completions
that

used larger injection volumes (104 to 105 cubic meters) and that
seismic productivity scales linearly with injection volume. Injection
pressure and rate have an insignificant association with seismic response.
Further findings suggest that geological factors play a prominent
role in seismic productivity, as evidenced by spatial correlations.
Together, volume and geological factors account for ~96% of the variability
in the induced earthquake rate near Fox Creek. This result is quantified
by a seismogenic indexmodified frequency-magnitude distribution,
providing a framework to forecast induced seismicity.

This study aims at understanding
the role of tidal level, speed, and direction in tsunami propagation
in highly energetic tidal channels. The main goal is to comprehend
whether tide-tsunami interactions enhance/reduce elevation, currents
speeds, and arrival times, when compared to pure tsunami models and
to simulations in which tides and tsunamis are linearly superimposed.
We designed various numerical experiments to compute the tsunami propagation
along Canal Chacao, a highly energetic channel in the Chilean Patagonia
lying on a subduction margin prone to megathrust earthquakes. Three
modeling approaches were implemented under the same

seismic scenario: a tsunami model with
a constant tide level, a series of six composite models in which independent
tide and tsunami simulations are linearly superimposed, and a series
of six tide-tsunami nonlinear interaction models (full models). We found
that hydrodynamic patterns differ significantly among approaches, being
the composite and full models sensitive to both the tidal phase at which
the tsunami is triggered and the local depth of the channel. When compared
to full models, composite models adequately predicted the maximum surface
elevation, but largely overestimated currents. The amplitude and arrival
time of the tsunami-leading wave computed with the full model was found
to be strongly dependent on the direction of the tidal current and less
responsive to the tide level and the tidal current speed. These outcomes
emphasize the importance of addressing more carefully the interactions
of tides and tsunamis on hazard assessment studies.

Geothermal heat flux (GHF) is a
crucial boundary condition for making accurate predictions of ice
sheet mass loss, yet it is poorly known in Greenland due to inaccessibility
of the bedrock. Here we use a machine learning algorithm on a large
collection of relevant geologic features and global GHF measurements
and produce a GHF

map of Greenland
that we argue is within ~15% accuracy. The main features of our predicted
GHF map include a large region with high GHF in central-north Greenland
surrounding the NorthGRIP ice core site, and hot spots in the Jakobshavn
Isbræ catchment, upstream of Petermann Gletscher, and near the
terminus of Nioghalvfjerdsfjorden glacier. Our model also captures the
trajectory of Greenland movement over the Icelandic plume by predicting
a stripe of elevated GHF in central-east Greenland. Finally, we show
that our model can produce substantially more accurate predictions if
additional measurements of GHF in Greenland are provided.

In a context of global change and
increasing anthropic pressure on the environment, monitoring the Earth
system and its evolution has become one of the key missions of geosciences.
Geodesy is the geoscience that measures the geometric shape of the
Earth, its orientation in space, and gravity field. Time-variable
gravity, because of its high accuracy, can be used to build an enhanced
picture and understanding of the

changing Earth. Ground-based gravimetry
can determine the change in gravity related to the Earth rotation fluctuation,
to celestial body and Earth attractions, to the mass in the direct vicinity
of the instruments, and to vertical displacement of the instrument itself
on the ground. In this paper, we review the geophysical questions that
can be addressed by ground gravimeters used to monitor time-variable
gravity. This is done in relation to the instrumental characteristics,
noise sources, and good practices. We also discuss the next challenges
to be met by ground gravimetry, the place that terrestrial gravimetry
should hold in the Earth observation system, and perspectives and recommendations
about the future of ground gravity instrumentation.

Fusing satellite observations and
station measurements to estimate ground-level PM2.5 is promising for
monitoring PM2.5 pollution. A geo-intelligent approach, which incorporates
geographical correlation into an intelligent deep learning architecture,
is developed to estimate PM2.5. Specifically, it considers geographical
distance and spatiotemporally correlated PM2.5 in

a deep belief network
(denoted as Geoi-DBN). Geoi-DBN can capture the essential features associated
with PM2.5 from latent factors. It was trained and tested with data
from China in 2015. The results show that Geoi-DBN performs significantly
better than the traditional neural network. The out-of-sample cross-validation
R2 increases from 0.42 to 0.88, and RMSE decreases from 29.96 to 13.03
µg/m3. On the basis of the derived PM2.5 distribution, it is predicted
that over 80% of the Chinese population live in areas with an annual
mean PM2.5 of greater than 35 µg/m3. This study provides a new
perspective for air pollution monitoring in large geographic regions.

In the first months of 2007, the
Aysén region in southern Chile was affected by a crustal seismic
swarm. Its largest earthquake (Mw 6.2) occurred in April, and had
its epicenter in Aysén Fjord. Seismic intensities became so
high that hundreds of onshore mass movements were triggered, several
of which entered into the fjord, resulting in mass-transport deposits
(MTDs) preserved at the fjord bottom. Here we present a Holocene record
of paleo-earthquakes in the previously unstudied Patagonian fjordland
based on MTD stratigraphy. High-resolution seismic data retrieved
using two different seismic systems (sparker and TOPAS) reveal multiple

older MTDs on different stratigraphic
levels. Correlation of the seismic stratigraphy with sedimentological
data obtained from a long Calypso core (MD07-3117) allows conclusion
on the seismic origin of these deposits. Additionally, radiocarbon dating
permits constructing an age model, validated by tephrochronology, providing
an age for the different MTD levels. We thus present a highly detailed
paleoseismological history of the Aysén region, including at
least six major Holocene earthquakes, one of which is likely related
to a known megathrust earthquake. Other earthquakes are related to activity
of the Liquiñe-Ofqui Fault Zone (LOFZ), forming the main source
of seismic hazard in the area. We can infer a general average recurrence
time for LOFZ earthquakes of ~2,100 years in the vicinity of Aysén
Fjord with clustered events during the Early and Late Holocene. Finally,
we argue that cascading events (causal link between volcanic and seismic
events) may be a frequent phenomenon along the LOFZ.

We develop a methodology that combines
compressive sensing back-projection (CS-BP) and source spectral analysis
of teleseismic P waves to provide metrics relevant to earthquake dynamics
of large events. We improve the CS-BP method by an auto-adaptive source
grid refinement as well as a reference source adjustment technique
to gain better spatial and temporal resolution of the locations of
the radiated bursts. We also use a two-step source spectral analysis
based on i) simple theoretical Green's functions that include depth
phases and water reverberations and on ii) empirical P-wave Green's
functions. Furthermore, we propose a source spectrogram methodology
that provides

the temporal evolution of dynamic parameters such
as radiated energy and falloff rates. Bridging back-projection and
spectrogram analysis provides a spatial and temporal evolution of
these dynamic source parameters. We apply our technique to the recent
2015 Mw 8.3 megathrust Illapel earthquake (Chile). The results from
both techniques are consistent and reveal a depth-varying seismic
radiation that is also found in other megathrust earthquakes. The
low frequency content of the seismic radiation is located in the shallow
part of the megathrust, propagating unilaterally from the hypocenter
towards the trench while most of the high frequency content comes
from the downdip part of the fault. Interpretation of multiple rupture
stages in the radiation is also supported by the temporal variations
of radiated energy and falloff rates. Finally, we discuss the possible
mechanisms, either from pre-stress, fault geometry, and/or frictional
properties to explain our observables. Our methodology is an attempt
to bridge kinematic observations with earthquake dynamics.

We present a four-category classification
algorithm for the solar wind, based on Gaussian Process. The four
categories are the ones previously adopted in Xu and Borovsky (2015):
ejecta, coronal hole origin plasma, streamer belt origin plasma, and
sector reversal origin plasma. The algorithm is trained and tested
on a labeled portion of the OMNI data set. It uses seven inputs: the
solar wind speed Vsw, the temperature standard deviation sT, the sunspot
number R, the F10.7 index, the Alfven speed vA, the proton specific
entropy Sp, and the proton temperature Tp compared to a velocity-dependent
expected

temperature. The output of the Gaussian
Process classifier is a four-element vector containing the probabilities
that an event (one reading from the hourly averaged OMNI database) belongs
to each category. The probabilistic nature of the prediction allows
for a more informative and flexible interpretation of the results, for
instance, being able to classify events as undecided. The
new method has a median accuracy larger than 90% for all categories,
even using a small set of data for training. The Receiver Operating
Characteristic curve and the reliability diagram also demonstrate the
excellent quality of this new method. Finally, we use the algorithm
to classify a large portion of the OMNI data set, and we present for
the first time transition probabilities between different solar wind
categories. Such probabilities represent the climatological
statistics that determine the solar wind baseline.

We present the PINE (Plasma density
in the Inner magnetosphere Neural network-based Empirical) model -
a new empirical model for reconstructing the global dynamics of the
cold plasma density distribution based only on solar wind data and
geomagnetic indices. Utilizing the density database obtained using
the NURD (Neural-network-based Upper hybrid Resonance Determination)
algorithm for the period of 1 October 2012 to 1 July 2016, in conjunction
with solar wind data and geomagnetic indices, we develop a neural
network model that is capable of globally reconstructing the dynamics
of the cold plasma density distribution for 2=L=6 and all local times.
We validate and test the model by

measuring its performance
on independent data sets withheld from the training set and by comparing
the model-predicted global evolution with global images of He+ distribution
in the Earth's plasmasphere from the IMAGE Extreme UltraViolet (EUV)
instrument. We identify the parameters that best quantify the plasmasphere
dynamics by training and comparing multiple neural networks with different
combinations of input parameters (geomagnetic indices, solar wind data,
and different durations of their time history). The optimal model is
based on the 96?h time history of Kp, AE, SYM-H, and F10.7 indices.
The model successfully reproduces erosion of the plasmasphere on the
nightside and plume formation and evolution. We demonstrate results
of both local and global plasma density reconstruction. This study illustrates
how global dynamics can be reconstructed from local in situ observations
by using machine learning techniques.

Ocean basins record the life history
of a tectonic plateits creation at a mid-ocean ridge, its thickening
over time, and its consumption at a subduction zone. The movement
of tectonic plates is possible because the lithosphere, Earth's stiff
outermost shell, slides on top of a weak asthenosphere. Despite its
fundamental role in facilitating plate tectonics, the nature of the

lithosphere-asthenosphere boundary is
poorly understood. The asthenosphere is on average warmer than the lithosphere,
but the temperature contrast alone may not provide the necessary viscosity
reduction. Previous work has also proposed a dehydrated lithosphere
and damp asthenosphere (1), and a solid lithosphere and partially molten
asthenosphere (2). On page 1593 of this issue, Takeuchi et al. (3) present
an analysis of aftershocks of the 2011 Tohoku earthquake and show how
the attenuation of seismic waves has a different frequency response
in the lithosphere versus the asthenosphere.

In a context of global change and
increasing anthropic pressure on the environment, monitoring the Earth
system and its evolution has become one of the key missions of geosciences.
Geodesy is the geoscience that measures the geometric shape of the
Earth, its orientation in space, and gravity field. Time-variable
gravity, because of its high accuracy, can be used to build an enhanced
picture and understanding of the changing Earth. Ground-based gravimetry
can

determine
the change in gravity related to the Earth rotation fluctuation, to
celestial body and Earth attractions, to the mass in the direct vicinity
of the instruments, and to vertical displacement of the instrument itself
on the ground. In this paper, we review the geophysical questions that
can be addressed by ground gravimeters used to monitor time-variable
gravity. This is done in relation to the instrumental characteristics,
noise sources, and good practices. We also discuss the next challenges
to be met by ground gravimetry, the place that terrestrial gravimetry
should hold in the Earth observation system, and perspectives and recommendations
about the future of ground gravity instrumentation.

Since 1990, nearly one million people
have died from the impacts of earthquakes. Reducing those impacts
requires building a local seismic culture in which residents are aware
of earthquake risks and value efforts to mitigate harm. Such efforts

include earthquake early
warning (EEW) systems that provide seconds to minutes notice of pending
shaking. Recent events in Mexico provide an opportunity to assess performance
and perception of an EEW system and highlight areas for further improvement.
We have learned that EEW systems, even imperfect ones, can help people
prepare for earthquakes and build local seismic culture, both beneficial
in reducing earthquake-related losses.

After an earthquake, the earliest
deformation signals are not expected to be carried by the fastest
(P) elastic waves but by the speed-of-light changes of the gravitational
field. However, these perturbations are weak and, so far, their detection
has not been accurate enough to fully understand

their
origins and to use them for a highly valuable rapid estimate of the
earthquake magnitude. We show that gravity perturbations are particularly
well observed with broadband seismometers at distances between 1000
and 2000 kilometers from the source of the 2011, moment magnitude 9.1,
Tohoku earthquake. We can accurately model them by a new formalism,
taking into account both the gravity changes and the gravity-induced
motion. These prompt elastogravity signals open the window for minute
time-scale magnitude determination for great earthquakes.

Comparison of pre-event geodetic
and geologic rates in three large-magnitude (Mw=7.6-7.9) strike-slip
earthquakes reveals a wide range of behaviors. Specifically, geodetic
rates of 26-28 mm/yr for the North Anatolian fault along the 1999
MW=7.6 Izmit rupture are ∼40% faster than Holocene geologic rates.
In contrast, geodetic rates of 6-8 mm/yr along the Denali fault prior
to the 2002 MW=7.9 Denali earthquake are only half as fast as the
latest Pleistocene-Holocene geologic rate of 12 mm/yr. In the third
example where a sufficiently long pre-earthquake geodetic

time series exists, the geodetic and
geologic rates along the 2001 MW=7.8 Kokoxili rupture on the Kunlun
fault are approximately equal at 11 mm/yr. These results are not readily
explicable with extant earthquake cycle modeling, suggesting that they
may instead be due to some combination of regional kinematic fault interactions,
temporal variations in the strength of lithospheric-scale shear zones,
and/or variations in local relative plate motion rate. Whatever the
exact causes of these variable behaviors, these observations indicate
that either the ratio of geodetic to geologic rates before an earthquake
may not be diagnostic of the time to the next earthquake, as predicted
by many rheologically based geodynamic models of earthquake cycle behavior,
or that different behaviors characterize different fault systems in
a manner that is not yet understood or predictable.

This paper presents a single-layer
travelling-wave antenna (TWA) that is based on composite right/left-handed
(CRLH)-metamaterial (MTM) transmission-line (TL) structure, which
is implemented by using a combination of inter-digital capacitors
and dual-spiral inductive slots. By embedding dual-spiral inductive
slots inside

the
CRLH MTM-TL results in a compact TWA. Dimensions of the proposed CRLH
MTM-TL TWA is 21.5×30.0 mm2 or 0.372λ0×0.520λ0 at 5.2 GHz (center
frequency). The fabricated TWA operates over 1.8–8.6 GHz with a fractional
bandwidth greater than 120%, and it exhibits a peak gain and radiation
efficiency of 4.2 dBi and 81%, respectively, at 5 GHz. By avoiding the
use of lumped components, via-holes or defected ground structures (DGS),
the proposed TWA design is economic for mass production as well as easy
to integrate with wireless communication systems.

The Valparaiso 2017 sequence occurred
in the Central Chile megathrust, an active zone where the last mega-earthquake
occurred in 1730. Intense seismicity started 2 days before the Mw
6.9 mainshock, a slow trenchward movement was observed in the coastal
GPS antennas and was accompanied by foreshocks and repeater-

type seismicity. To characterize the rupture
process of the mainshock, we perform a dynamic inversion using the strong-motion
records and an elliptical patch approach. We suggest that a slow slip
event preceded and triggered the Mw 6.9 earthquake, which ruptured an
elliptical asperity (semiaxis of 10 km and 5 km, with a subshear rupture,
stress drop of 11.71 MPa, yield stress of 17.21 MPa, slip weakening
of 0.65 m, and kappa value of 1.98). This earthquake could be the beginning
of a long-term nucleation phase to a major rupture, within the highly
coupled Central Chile zone where a megathrust earthquake like 1730 is
expected.

For decades, scientists have charted
tiny fluctuations in the length of Earth's day: Gain a millisecond
here, lose a millisecond there. Last week at the annual meeting of
the Geological Society of America in Seattle, Washington, two geophysicists
argued that these minute changes

could
be correlated with the timing of major earthquakes—and potentially
help forecast them. During the past 100 years, Earth's slowdowns have
matched surprisingly well with periods of global increases in the frequency
of magnitude-7 and larger earthquakes. Each spike happens well after
the slowdown, offering a 5-year heads up on future quakes. So far, researchers
have only fuzzy ideas about how changes in Earth's molten iron core
might cause this pattern, but they say the finding is too provocative
to ignore.

Increasing attention has been paid
to the prediction of earthquakes with data mining techniques during
the last decade. Several works have already proposed the use of certain
features serving as inputs for supervised classifiers. However, they
have been successfully used without any further transformation so
far. In this work, the use of principal component analysis (PCA) to
reduce data dimensionality and generate new datasets is proposed.
In particular, this step is inserted in a successfully already used
methodology to predict earthquakes. Tokyo, one of the cities mostly
threatened by large earthquakes occurrence in Japan, is studied. Several
well-known classifiers combined with PCA have been used. Noticeable
improvement in the results is reported.

Large earthquakes on subduction
zone plate boundary megathrusts result from intervals of strain accumulation
and release. The mechanism diversity and spatial distribution of moderate-size
aftershocks is influenced by the mainshock rupture depth extent. Mainshocks
that rupture across the shallow megathrust to near the trench have
greater intraplate aftershock faulting diversity

than
events with rupture confined to deeper portions of the megathrust. Diversity
of intraplate aftershock faulting also increases as the size of the
mainshock approaches the largest size event to have ruptured that region
of the megathrust. Based on these tendencies, we identify “breakthrough”
ruptures as those involving shallow rupture of the megathrust with volumetrically
extensive elastic strain drop around the plate boundary that allows
activation of diverse intraplate faulting influenced by long-term ambient
deformation stresses. In contrast, homogeneity of the aftershock faulting
mechanisms indicates only partial release of elastic strain energy and
remaining potential for another large rupture.

Mars is believed to have possessed
a dynamo that ceased operating approximately 4 Ga ago, although the
exact time is still under debate. The scope of this study is to constrain
the possible timing of its cessation by studying the magnetization
signatures of craters. The study uses the latest available model of
the lithospheric magnetic field of Mars, which is based on Mars Global
Surveyor data. We tackle the problem of non-uniqueness that characterises
the inversion of magnetic field data for the magnetization by inferring
only the visible part of the magnetization, i.e., the part of the
magnetization that gives rise to

the observed magnetic field. Further on,
we demonstrate that a zero visible magnetization is a valid proxy for
the entire magnetization being zero under the assumption of a magnetization
distribution of induced geometry. This assumption holds for craters
whose thermoremanent magnetization has not been significantly altered
since its acquisition. Our results show that the dynamo shut off after
the impacts that created the Acidalia and SE Elysium basins and before
the crust within the Utopia basin cooled below its magnetic blocking
temperature. Accounting for the age uncertainties in the dating of these
craters, we estimate that the dynamo shut off at an N(300) crater retention
age of 2.5-3.2 or an absolute model age of 4.12 - 4.14 Ga. Moreover,
the Martian dynamo may have been weaker in its early stage, which if
true implies that the driving mechanism of the Martian dynamo was not
the same throughout its history.

Earthquake-induced landslide (EQIL)
inventories are essential tools to extend our knowledge of the relationship
between earthquakes and the landslides they can trigger. Regrettably,
such inventories are difficult to generate and therefore scarce, and
the available ones differ in terms of their quality and level of completeness.
Moreover, access to existing EQIL inventories is currently difficult
because there is no centralized database. To address these issues,
we compiled EQIL inventories from around the globe based on an extensive
literature study. The database contains information on 363 landslide-triggering
earthquakes and includes 66 digital landslide inventories. To make
these data openly available, we created a repository to host the digital

inventories
that we have permission to redistribute through the U.S. Geological
Survey ScienceBase platform. It can grow over time as more authors contribute
their inventories. We analyze the distribution of EQIL events by time
period and location, more specifically breaking down the distribution
by continent, country and mountain region. Additionally, we analyze
frequency distributions of EQIL characteristics, such as the approximate
area affected by landslides, total number of landslides, maximum distance
from fault rupture zone, and distance from epicenter when the fault
plane location is unknown. For the available digital EQIL inventories,
we examine the underlying characteristics of landslide size, topographic
slope, roughness, local relief, distance to streams, peak ground acceleration,
peak ground velocity, and Modified Mercalli Intensity. Also, we present
an evaluation system to help users assess the suitability of the available
inventories for different types of EQIL studies and model development.

The largest observed earthquakes
occur on subduction interfaces and frequently cause widespread damage
and loss of life. Understanding the rupture behavior of megathrust
events is crucial for earthquake rupture physics, as well as for earthquake
early-warning systems. However, the large variability in behavior
between

individual events seemingly defies a description
with a simple unifying model. Here we use three source time function
(STF) data sets for subduction zone earthquakes, with moment magnitude
Mw = 7, and show that such large ruptures share a typical universal
behavior. The median STF is scalable between events with different sizes,
grows linearly, and is nearly triangular. The deviations from the median
behavior are multiplicative and Gaussian—that is, they are proportionally
larger for larger events. Our observations suggest that earthquake magnitudes
cannot be predicted from the characteristics of rupture onsets.

We apply machine learning to data
sets from shear laboratory experiments, with the goal of identifying
hidden signals that precede earthquakes. Here we show that by listening
to the acoustic signal emitted by a laboratory fault, machine learning
can predict the time remaining

before
it fails with great accuracy. These predictions are based solely on
the instantaneous physical characteristics of the acoustical signal
and do not make use of its history. Surprisingly, machine learning identifies
a signal emitted from the fault zone previously thought to be low-amplitude
noise that enables failure forecasting throughout the laboratory quake
cycle. We infer that this signal originates from continuous grain motions
of the fault gouge as the fault blocks displace. We posit that applying
this approach to continuous seismic data may lead to significant advances
in identifying currently unknown signals, in providing new insights
into fault physics, and in placing bounds on fault failure times.

indicate that fracture deformation in
the normal direction reverses as the pressure decreases, e.g., at the
end of stimulation. We hypothesize that this normal closure of fractures
enhances pressure propagation away from the injection region and significantly
increases the potential for post-injection seismicity. To test this
hypothesis, hydraulic stimulation is modeled by numerically coupling
flow in the fractures and matrix, fracture deformation and matrix deformation
for a synthetic reservoir in which the flow and mechanics are strongly
affected by a complex three-dimensional fracture network. The role of
the normal closure of fractures is verified by comparing simulations
conducted with and without the normal closure effect.

Recent observations suggested that
ionospheric anomalies appear immediately before large earthquakes
with moment magnitudes (Mw) of 8.2 or more. Do similar phenomena precede
smaller earthquakes? Here we answer this question by analyzing vertical
total electron contents (VTEC) observed near the epicenters before
and after 32 earthquakes with Mw7.0–8.0 using data from nearby Global
Navigation Satellite Systems stations. To detect anomalies, we

defined
the reference curves to fit the observed VTEC and considered the departure
from the curves as anomalies. In estimating the reference curves, we
excluded time windows, prescribed for individual earthquakes considering
Mw, possibly affected by earthquakes. We validated the method using
synthetic VTEC data assuming both preseismic, coseismic, and postseismic
anomalies. Out of the 32 Mw7.0–8.0 earthquakes, eight earthquakes
showed possible preseismic anomalies starting 10–20 min before earthquakes.
For earthquakes of this Mw range, we can observe preseismic ionospheric
changes probably when the background VTEC is large, say 50 TECU (total
electron content unit, 1 TECU = 1016 el m-2) or more.

In a context of global change and
increasing anthropic pressure on the environment, monitoring the Earth
system and its evolution has become one of the key missions of geosciences.
Geodesy is the geoscience that measures the geometric shape of the
Earth, its orientation in space, and gravity field.Time-variable gravity,
because of its high accuracy, can be used to build an enhanced picture
and understanding of the changing Earth. Ground-based gravimetry can

determine the change in gravity related
to the Earth rotation fluctuation, to celestial-body and Earth attractions,
to the mass in the direct vicinity of the instruments, and vertical
displacement of the instrument itself on the ground.
In this paper, we review the geophysical questions that can be addressed
byground gravimeters used to monitor time-variable gravity. This is
done in relation to the instrumental characteristics, noise sources
and good practices. We also discuss the next challenges to be met by
ground gravimetry,the place that terrestrial gravimetry should hold
in the Earth observation system, and perspectives and recommendations
about the future of ground gravity instrumentation.

Field, geochemical, and geochronological
data show that the southern segment of the ancestral Cascades arc
advanced into the Oregon back-arc region from 30 to 20 Ma. We attribute
this event to thermal uplift of the Farallon slab by the

Yellowstone mantle
plume, with heat diffusion, decompression, and the release of volatiles
promoting high-K calc-alkaline volcanism throughout the back-arc region.
The greatest degree of heating is expressed at the surface by a broad
ENE-trending zone of adakites and related rocks generated by melting
of oceanic crust from the Farallon slab. A hiatus in eruptive activity
began at ca. 22–20 Ma but ended abruptly at 16.7 Ma with renewed volcanism
from slab rupture occurring in two separate regions. The eastern rupture
resulted in the extrusion of Steens Basalt

We numerically compute seismoelectric
wavefields generated at a fluid/porous medium interface by an explosive
source in the fluid. Our numerical experiments show that electromagnetic
(EM) signals accompanying the P, S, and interface waves can be observed
at receivers located in the fluid regions near the interface. Such
accompanying EM signals are produced by the inhomogeneous EM waves
that are generated by the seismic waves at the interface and their
amplitudes decrease with the distance from interface. Under the excitation
of an explosive

source whose strength is within the capability
of industry air guns, electric and magnetic fields that accompany the
Scholte wave are on the order of 1 μV/m and 0.01 nT, respectively.
This means that the EM signals arising from the electrokinetic effect
at an ocean bottom are detectable and suggest that it is possible to
measure the EM signals during marine seismic explorations to study the
properties of the seafloor material. EM signals that accompany the P,
S, and interface waves are also observed in the porous medium region
near the interface. Component analysis shows that they contain contributions
from multiple modes of waves, among which the slow compressional wave
contributes significantly to the vertical electric field, leading to
a much stronger vertical electric field than the horizontal electric
field during the passage of a seismic wave along the interface.

Landslides are the second most important
cause of tsunamis after earthquakes, and their potential for generating
large tsunamis depend on the slide process. Among the world's largest
submarine landslides is the Storegga Slide that generated a large
tsunami over an ocean-wide scale, while no traces of a tsunami generated
from the similar and nearby Trænadjupet Slide have been found. Previous
models for such landslide tsunamis have not been able to capture

the complexity of the landslide processes
and are at odds with geotechnical and geomorphological data that reveal
retrogressive landslide development. The tsunami generation from these
massive events are here modeled with new methods that incorporate complex
retrogressive slide motion. We show that the tsunamigenic strength is
closely related to the retrogressive development and explain, for the
first time, why similar giant landslides can produce very different
tsunamis, sometimes smaller than anticipated. Because these slide mechanisms
are common for submarine landslides, modeling procedures for dealing
with their associated tsunamis should be revised.

We investigate possible biasing
effects of inaccurate timing corrections on teleseismic P-wave back-projection
imaging of large earthquake ruptures. These errors occur because empirically-estimated
time shifts based on aligning P-wave first arrivals are exact only
at the hypocenter and provide approximate corrections for other parts
of the rupture. Using the Japan subduction zone as a test region,
we analyze 46 M6–7 earthquakes over a ten-year period, including
many aftershocks of the 2011 M9 Tohoku earthquake, performing waveform
cross-correlation of their initial P-wave arrivals to obtain hypocenter
timing

corrections to global seismic stations.
We then compare back-projection images for each earthquake using its
own timing corrections with those obtained using the time corrections
from other earthquakes. This provides a measure of how well sub-events
can be resolved with back-projection of a large rupture as a function
of distance from the hypocenter. Our results show that back-projection
is generally very robust and that the median sub-event location error
is about 25 km across the entire study region (∼700 km). The back-projection
coherence loss and location errors do not noticeably converge to zero
even when the event pairs are very close (<20 km). This indicates
that most of the timing differences are due to 3D structure close to
each of the hypocenter regions, which limits the effectiveness of attempts
to refine back-projection images using aftershock calibration, at least
in this region.

This paper proposes a stochastic
approach to model the earthquake uncertainties in terms of the rupture
location and the slip distribution for a future event, with an expected
earthquake magnitude. Once the statistical properties of earthquake
uncertainties are described, they are then propagated into the tsunami
response and the inundation at assessed coastal areas. The slip distribution
is modeled as a random field within a nonrectangular rupture area.
The Karhunen-Lòeve (K-L) expansion method is used to generate samples
of the random slip, and a translation model is employed to obtain
target probability properties. A strategy is developed to specify
the accuracy of the random samples in

terms of numbers of subfaults of the rupture
area and the truncation of the K-L expansion. The propagation of uncertainty
into the tsunami response is performed by means of a Stochastic Reduced
Order Model. To illustrate the methodology, we investigated a study
case in north Chile. We first demonstrate that the stochastic approach
generates consistent earthquake samples with respect to the target probability
properties. We also show that the results obtained from SROM are more
accurate than those obtained with classic Monte Carlo simulations. To
validate the methodology, we compared the simulated tsunamis and the
tsunami records for the 2014 Chilean earthquake. Results show that leading
wave measurements fall within the tsunami sample space. At later times,
however, there are mismatches between measured data and the simulated
results, suggesting that other sources of uncertainties are as relevant
as the uncertainty of earthquakes.

In April 2017, a sequence
of earthquakes offshore Valparaíso, Chile, raised concerns
of a
potential megathrust earthquake in the near future. The largest
event in the 2017 sequence was a M6.9 on
24 April, seemingly colocated with the last great-sized earthquake
in the region—a M8.0 in March 1985. The
history of large earthquakes in this region shows significant
variation in rupture size and extent, typically
highlighted by a juxtaposition of large ruptures interspersed
with smaller magnitude sequences. We show
that the 2017 sequence ruptured an area between the two main
slip patches during the 1985 earthquake,
rerupturing a patch that had previously slipped during the October
1973 M6.5 earthquake sequence. A
significant gap in historic ruptures exists directly to the
south of the 2017 sequence, with large enough
moment deficit to host a great-sized earthquake in the near
future, if it is locked.

When a large earthquake occurs near
an active volcano, there is often concern that volcanic eruptions
may be triggered by the earthquake. In this study, recently accumulated,
reliable data were analyzed to quantitatively evaluate the probability
of the occurrence of new eruptions of volcanoes located near the epicenters
of large earthquakes. For volcanoes located within 200 km of large
earthquakes of magnitude 7.5 or

greater, the eruption occurrence probability
increases by approximately 50% for 5 years after the earthquake origin
time. However, no significant increase in the occurrence probability
of new eruptions was observed at distant volcanoes or for smaller earthquakes.
The present results strongly suggest that new eruptions are likely triggered
by static stress changes and/or strong ground motions caused by nearby
large earthquakes. This is not similar to the previously presented evidence
that volcanic earthquakes at distant volcanoes are remotely triggered
by surface waves generated by large earthquakes.

Earthquakes induced by natural gas
extraction from the Groningen reservoir, the Netherlands, put local
communities at risk. Responsible operation of a reservoir whose gas
reserves are of strategic importance to the country requires understanding
of the link between extraction and earthquakes. We synthesize observations
and a model for Groningen seismicity to produce forecasts for felt
seismicity (M > 2.5) in the period

February 2017 to 2024. Our model accounts
for poroelastic earthquake triggering and rupture on the 325 largest
reservoir faults, using an ensemble approach to model unknown heterogeneity
and replicate earthquake statistics. We calculate probability distributions
for key model parameters using a Bayesian method that incorporates the
earthquake observations with a nonhomogeneous Poisson process. Our analysis
indicates that the Groningen reservoir was not critically stressed prior
to the start of production. Epistemic uncertainty and aleatoric uncertainty
are incorporated into forecasts for three different future extraction
scenarios. The largest expected eart hquake was similar for all scenarios,
with a 5% likelihood of exceeding M 4.0.

Cosmochemical and geochemical studies
suggest sulfur (S) as a light alloying element in the iron-rich cores
of telluric planets, but there is no report of sulfur's alloying effect
on the electrical and thermal transport properties of iron (Fe); a
subject that is closely related to the dynamo

action and thermal evolution of planetary
cores. We measured the electrical resistivity of hexagonal-closed-packed
(hcp) structured Fe alloy containing 3 wt. % silicon (Si) and 3 wt.
% S up to 110 GPa at 300 K. Combined with the reported resistivities
of hcp Fe and hcp Fe-Si alloy, we determined the impurity resistivity
of S in a hcp Fe matrix at high pressures. The obtained impurity resistivity
of S is found to be smaller than that of Si. Therefore, S is a weaker
influence on the conductivity of Fe alloy, even if S is a major light
element in the planetary cores.

GNSS-based earthquake early warning
(EEW) algorithms estimate fault-finiteness and unsaturated moment
magnitude for the largest, most damaging earthquakes. Because large
events are infrequent, algorithms are not regularly exercised and
insufficiently tested on few available datasets. We use 1300 realistic,
time-dependent, synthetic earthquakes on the Cascadia megathrust to
rigorously test the

Geodetic Alarm System.
Solutions are reliable once six GNSS stations report static offsets,
which we require for a “first alert.” Median magnitude and length
errors are -0.15±0.24 units and -31±40% for the first alert, and -0.04±0.11
units and +7±31% for the final solution. We perform a coupled test
of a seismic-geodetic EEW system using synthetic waveforms for a Mw8.7
scenario. Seismic point-source solutions result in severely underestimated
PGA. Geodetic finite-fault solutions provide more accurate predictions
at larger distances, thus increasing warning times. Hence, GNSS observations
are essential in EEW to accurately characterize large (out-of-network)
events and correctly predict ground motion.

much thicker root (>250 kilometers)
and a gradual lithosphere-asthenosphere transition, consistent with
a thermal definition. We modeled SS precursor waveforms from continental
interiors and found a 7 to 9% velocity drop at depths of 130 to 190
kilometers. The discontinuity depth is well correlated with the origin
depths of diamond-bearing xenoliths and corresponds to the transition
from coarse to deformed xenoliths. At this depth, the xenolith-derived
geotherm also intersects the carbonate-silicate solidus, suggesting
that partial melt defines the plate boundaries beneath the continental
interior.

Real-time ground motion alerts,
as can be provided by Earthquake Early Warning (EEW) systems, need
to be both timely and sufficiently accurate to be useful. Yet how
timely and how accurate the alerts of existing EEW algorithms are
is often poorly understood. In part, this is because EEW algorithm
performance is usually evaluated not in terms of ground motion prediction
accuracy and timeliness but in terms of other metrics (e.g., magnitude
and location estimation errors), which do not directly reflect the
usefulness of the alerts from an end user perspective. Here we attempt
to

identify a suite
of metrics for EEW algorithm performance evaluation that directly quantify
an algorithm's ability to identify target sites that will experience
ground motion above a critical (user-defined) ground motion threshold.
We process 15,553 recordings from 238 earthquakes with M > 5 (mostly
from Japan and southern California) in a pseudo-real-time environment
and investigate two end-member EEW methods. We use the metrics to highlight
both the potential and limitations of the two algorithms and to show
under which circumstances useful alerts can be provided. Such metrics
could be used by EEW algorithm developers to convincingly demonstrate
the added value of new algorithms or algorithm components. They can
complement existing performance metrics that quantify other relevant
aspects of EEW algorithms (e.g., false event detection rates) for a
comprehensive and meaningful EEW performance analysis.

Agosto de 2017How variable is the number of triggered aftershocks?Authors: D. Marsan and A. Helmstetter
Link: Click here

Aftershock activity depends at first
order on the main shock magnitude but also shows important fluctuations
between shocks of equal magnitude. We here investigate these fluctuations,
by quantifying them and by relating them to the main shock stress
drop and other variables, for

southern California earthquakes. A method
is proposed in order to only count directly triggered aftershocks, rather
than secondary aftershocks (i.e., triggered by previous aftershocks),
and to only quantify fluctuations going beyond the natural Poisson variability.
Testing of the method subjected to various model errors allows to quantify
its robustness. It is found that these fluctuations follow a distribution
that is well fitted by a lognormal distribution, with a coefficient
of variation of about 1.0 to 1.1. A simple model is proposed to relate
this observed dependence to main shock stress drop variability.

Continents have tolerated billions
of years of tectonic stresses and disfigurement, yet they continue
to survive. Compared with their oceanic counterpart, where a sinking
demise is an almost certainty, continents and their internal cores,
or cratons, are much thicker (>175 km), older (>2 billion years),
colder, and more buoyant. However, their basic attributes, such as
size and shape, are

a still a matter
of debate because of large uncertainties in deceivingly straightforward,
but entirely complicated, measurements. Continental cratons are rigid
bodies composed of both crust and mantle, and their thickness was thought
to be related to temperature and extend to depths of 250 to 350 km.
On page 580 of this issue, Tharimena et al. (1) use reflections of seismic
waves within the cratons to constrain their thickness globally. The
strength of the reflections suggests that the base of the cratonic plate
is defined by a partial melt of carbon-laced silicate mantle, not temperature.

The amplitude asymmetry and initial
polarity of seismic induced ionospheric perturbations around the epicenter
are considered to be important in providing information about the
rupture propagation and related vertical surface deformation. To comprehend
this, we study ionospheric perturbations related to the 12 May 2015,
Mw 7.3 Nepal earthquake. We model the

coseismic slip associated with the event
using the interferometric synthetic aperture radar derived surface deformation
data. The ionospheric perturbations associated with the obtained surface
deformation are explained in terms of rupture propagation, favorable
geomagnetic field-wave coupling, and satellite geometry effects. We
discuss the effects of phase cancelation on the perturbation evolution
for various receiver satellite line-of-sight configurations invoking
an elementary version of satellite geometry factor. The present study
thus elucidates further the role of nontectonic forcing mechanisms while
identifying ground source pattern using the associated ionospheric perturbations.

Ultralow-velocity zones are localized
regions of extreme material properties detected seismologically at
the base of Earth's mantle. Their nature and role in mantle dynamics
are poorly understood. We used shear waves diffracted at the core-mantle
boundary to

illuminate the root of the Iceland plume
from different directions. Through waveform modeling, we detected a
large ultralow-velocity zone and constrained its shape to be axisymmetric
to a very good first order. We thus attribute it to partial melting
of a locally thickened, denser- and hotter-than-average layer, reflecting
dynamics and elevated temperatures within the plume root. Such structures
are few and far apart, and they may be characteristic of the roots of
some of the broad mantle plumes tomographically imaged within the large
low-shear-velocity provinces in the lower mantle.

The ultimate goal of volcanology
is forecasting eruptions. This task is particularly challenging at
calderas, where unrest is frequent, affects wider areas and its evidence
is often masked by the activity of hydrothermal systems. A recent
study has compiled a database on caldera unrest, derived from seismicity,
geodetic, gravity, and geochemical monitoring data at calderas worldwide,
from 1988 to 2014. Here we exploit this database, searching for the
most recurring features of unrest and, in turn, its possible dynamics.
In particular, we focus on (a) the duration of unrest at calderas;
(b) recurring patterns in unrest; (c) unrest episodes culminating
in eruptions, including time-predictability or size-predictability
and a

multivariate regression
analysis. Our analysis indicates that preeruptive unrest is shorter
than noneruptive unrest, particularly with open or semiplugged calderas,
calderas with mafic or mixed composition of past eruptive products,
or unrest driven by mafic magma; conversely, lack of data on preeruptive
unrest driven by felsic magma and/or at felsic or plugged calderas prevents
an analysis of these specific subsets. In addition, 72% of preeruptive
unrest lasts <10 months and shows high seismicity and degassing.
The remaining 28% (a) is essentially aseismic in calderas with open-conduit
(17%), or (b) lasts between 10 and 18 months, with seismicity and degassing,
constituting a longer-duration tail of the preeruptive unrest with seismicity
and degassing (11%). Surface deformation is not always reliable to characterize
preeruptive unrest. Our analysis suggests that magma may withstand only
a limited period of “eruptability,” before becoming stored in the
upper crust.

Copahue volcano straddling the edge
of the Agrio-Caviahue caldera along the Chile-Argentina border in
the southern Andes has been in unrest since inflation began in late
2011. We constrain Copahue's source models with satellite and airborne
interferometric synthetic aperture radar (InSAR) deformation observations.
InSAR time series from descending track RADARSAT-2 and COSMO-SkyMed
data span the entire inflation period from 2011 to 2016, with their
initially high rates of 12 and 15 cm/yr, respectively, slowing only
slightly despite ongoing small eruptions through 2016. InSAR ascending
and descending track time series for the 2013–2016 time period

constrain a two-source compound dislocation
model, with a rate of volume increase of 13 × 106 m3/yr. They consist
of a shallow, near-vertical, elongated source centered at 2.5 km beneath
the summit and a deeper, shallowly plunging source centered at 7 km
depth connecting the shallow source to the deeper caldera. The deeper
source is located directly beneath the volcano tectonic seismicity with
the lower bounds of the seismicity parallel to the plunge of the deep
source. InSAR time series also show normal fault offsets on the NE flank
Copahue faults. Coulomb stress change calculations for right-lateral
strike slip (RLSS), thrust, and normal receiver faults show positive
values in the north caldera for both RLSS and normal faults, suggesting
that northward trending seismicity and Copahue fault motion within the
caldera are caused by the modeled sources. Together, the InSAR-constrained
source model and the seismicity suggest a deep conduit or transfer zone
where magma moves from the central caldera to Copahue's upper edifice.

On 25 December 2016, the Mw 7.6
Chiloé earthquake broke a plate boundary asperity in south central
Chile near the center of the rupture zone of the Mw 9.5 Valdivia earthquake
of 1960. To gain insight on decadal-scale deformation trends and their
relation with the Chiloé earthquake, we combine geodetic, teleseismic,
and regional seismological data. GPS velocities increased at continental
scale after the 2010

Maule earthquake, probably due to a readjustment
in the mantle flow and an apparently abrupt end of the viscoelastic
mantle relaxation following the 1960 Valdivia earthquake. It also produced
an increase in the degree of plate locking. The Chiloé earthquake occurred
within the region of increased locking, breaking a circular patch of
~15 km radius at ~30 km depth, located near the bottom of the seismogenic
zone. We propose that the Chiloé earthquake is a first sign of the
seismic reawakening of the Valdivia segment, in response to the interaction
between postseismic viscoelastic relaxation and changes of interseismic
locking between Nazca and South America.

To explore where earthquakes tend
to recur, we statistically investigated repeating earthquake catalogs
and background seismicity from different regions (Parkfield, Hayward,
Calaveras, and Chihshang Faults). We show that the location of repeating
earthquakes can be mapped using the spatial distribution of the seismic
a and b values

obtained from the
background seismicity. Molchan's error diagram statistically confirmed
that repeating earthquakes occur within areas with high a values (2.8–3.8)
and high b values (0.9–1.1) on both strike-slip and thrust fault segments.
However, no significant association held true for fault segments with
more complicated geometry or for wider areas with a complex fault network.
The productivity of small earthquakes responsible for high a and b values
may thus be the most important factor controlling the location of repeating
earthquakes. We inferred that the location of high creep rate in planar/listric
fault structures might be indicated by a values of ~3 and b values of
~1.

Recent observations suggested that
ionospheric anomalies appear immediately before large earthquakes
with moment magnitudes (Mw) of 8.2 or more. Do similar phenomena precede
smaller earthquakes? Here we answer this question by analyzing vertical
total electron contents (VTEC) observed near the epicenters before
and after 32 earthquakes with Mw7.0-8.0 using data from nearby Global
Navigation Satellite

System (GNSS) stations. To detect anomalies,
we defined the reference curves to fit the observed VTEC, and considered
the departure from the curves as anomalies. In estimating the reference
curves, we excluded time windows, prescribed for individual earthquakes
considering Mw, possibly affected by earthquakes. We validated the method
using synthetic VTEC data assuming both pre-, co- and postseismic anomalies.
Out of the 32 Mw7.0-8.0 earthquakes, 8 earthquakes showed possible preseismic
anomalies starting 10-20 minutes before earthquakes. For earthquakes
of this Mw range, we can observe preseismic ionospheric changes probably
when the background VTEC is large, say 50 TECU or more.

The Volcanic Explosivity Index (VEI)
5 eruption of the Puyehue-Cordón Caulle volcanic complex (PCC) in
central Chile, which began 4 June 2011, provides a rare opportunity
to assess the rapid transport and deposition of sulfate and ash from
a mid-latitude volcano to the Antarctic ice sheet. We present sulfate,
microparticle concentrations of fine-grained (~5 μm diameter) tephra,
and major oxide geochemistry, which document the depositional sequence
of volcanic products from the PCC eruption in West Antarctic snow
and shallow firn. From the depositional phasing and duration of ash
and sulfate peaks, we infer that transport occurred primarily through
the troposphere but that ash and sulfate transport

were decoupled. We
use Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT)
back-trajectory modeling to assess atmospheric circulation conditions
in the weeks following the eruption, and find that conditions favored
southward air parcel transport during 6-14 June and 4-18 July, 2011.
We suggest that two discrete pulses of cryptotephra deposition relate
to these intervals, and as such, constrain the sulfate transport and
deposition lifespan to the ~2-3 weeks following the eruption. Finally,
we compare PCC depositional patterns to those of prominent low- and
high-latitude eruptions in order to improve multiparameter-based efforts
to identify “unknown source” eruptions in the ice core record. Our
observations suggest that mid-latitude eruptions such as PCC can be
distinguished from explosive tropical eruptions by differences in ash/sulfate
phasing and in the duration of sulfate deposition, and from high-latitude
eruptions by differences in particle size distribution and in cryptotephra
geochemical composition.

On 16 September 2015, a Mw 8.3
earthquake ruptured the subduction zone offshore of Illapel, Chile,
generating an aftershock sequence with 14 Mw 6.0–7.0 events. A double
source W phase moment tensor inversion consists of a Mw 7.2 subevent
and the main Mw 8.2 phase. We determine two slip models for the mainshock,
one using teleseismic broadband waveforms and the other using static
GPS and InSAR surface displacements, which indicate high slip north
of the epicenter and west-northwest of the epicenter near the oceanic
trench. These models and slip distributions published in other studies
suggest spatial slip uncertainties of ~25 km and have

peak slip values that vary by a factor
of 2. We relocate aftershock hypocenters using a Bayesian multiple-event
relocation algorithm, revealing a cluster of aftershocks under the Chilean
coast associated with deep (20–45 km depth) mainshock slip. Less vigorous
aftershock activity also occurred near the trench and along strike of
the main aftershock region. Most aftershocks are thrust-faulting events,
except for normal-faulting events near the trench. Coulomb failure stress
change amplitudes and signs are uncertain for aftershocks collocated
with deeper mainshock slip; other aftershocks are more clearly associated
with loading from the mainshock. These observations reveal a frictionally
heterogeneous interface that ruptured in patches at seismogenic depths
(associated with many aftershocks) and with homogeneous slip (and few
aftershocks) up to the trench. This event likely triggered seismicity
separate from the main slip region, including along-strike events on
the megathrust and intraplate extensional events.

To explore where earthquakes tend
to recur, we statistically investigated repeating earthquake catalogs
and background seismicity from different regions (Parkfield, Hayward,
Calaveras, and Chihshang Faults). We show that the location of repeating
earthquakes can be mapped using the spatial distribution of the seismic
a and b values

obtained from the
background seismicity. Molchan's error diagram statistically confirmed
that repeating earthquakes occur within areas with high a values (2.8–3.8)
and high b values (0.9–1.1) on both strike-slip and thrust fault segments.
However, no significant association held true for fault segments with
more complicated geometry or for wider areas with a complex fault network.
The productivity of small earthquakes responsible for high a and b values
may thus be the most important factor controlling the location of repeating
earthquakes. We inferred that the location of high creep rate in planar/listric
fault structures might be indicated by a values of ~3 and b values of
~1.

Once the terrestrial planets had
mostly completed their assembly, bombardment continued by planetesimals
left over from accretion. Highly siderophile element (HSE) abundances
in Mars' mantle imply that its late accretion supplement was 0.8 wt
%; Earth and the Moon obtained an additional 0.7 wt % and 0.02 wt
%, respectively.

The disproportionately high Earth/Moon
accretion ratio is explicable by stochastic addition of a few remaining
Ceres-sized bodies that preferentially targeted Earth. Here we show
that Mars' late accretion budget also requires a colossal impact, a
plausible visible remnant of which is the emispheric dichotomy. The
addition of sufficient HSEs to the Martian mantle entails an impactor
of at least 1200 km in diameter to have struck Mars before ~4430 Ma,
by which time crust formation was well underway. Thus, the dichotomy
could be one of the oldest geophysical features of the Martian crust.
Ejected debris could be the source material for its satellites.